Whether the assumption is correct or not, some fans attended Monday's show thinking this may be the last time

Given that concertgoers have been paying almost three times as much to see the Rolling Stones on their current tour as they did in 2005-06 (about $355 per ticket compared with $135 on average on the previous tours, according to Pollstar), one might draw this conclusion about the band:

Time is on their side.

Yet there’s a reason people are paying $600 for floor and lower-stands arena seats, and it’s not that they expect the band to perform almost three times as well as they did seven years ago. Rather, it’s this simple point:

“I’ve been singing that song for weeks,” said Marla Neff, 50, who was wearing a British flag dress and carrying a Rolling Stones lunchbox while up from St. Louis for Monday’s third and final United Center show. “They’re getting pretty old, like the rest of us. And it’s a long time between tours.”

The Stones first played in Chicago in late 1964 at the Arie Crown Theater, though they’d already recorded at the blues-steeped Chess Studios by that time. Would Monday’s concert close the book on almost 50 years of Stones shows in this city, thus providing one of those opportunities to say “I was there” for the rest of your life?

“I think it is history,” said Nick Harrison, 22, who attended the show with his brother Matt, 23, and father, Ed, 47, from the Western suburbs. “It’s their last show in Chicago.”

Brian Setzer, whose band the Stray Cats opened for the Stones in the early 1980s, was seated in the arena’s lower level with his 16- and 9-year-old daughters, whom he’d brought up from Nashville.

“When Elvis played my dad couldn’t afford the tickets,” said Setzer, who thus missed seeing the King. “I want to make sure they see the Stones.”

“I figured this was our first and maybe last chance,” said Michael Burris, 25, who flew up from Jupiter, Fla., with his wife, Katie, to celebrate their anniversary by spending far more than they’d ever spent on concert tickets: $600 apiece for floor seats.

“They are the last of the greatest rock ‘n’ roll bands of all time, and out of our favorites, they’re probably the last one we’re going to see,” said Katie Burris, 24. “Can’t see the Beatles, Queen, Led Zeppelin.”

But then, you never know with the Stones — they could be back. Who’d have thought Keith Richards would have lasted this long in the first place? And, as veteran rock critic/author Dave Marsh told me for an article four years ago: “I don't think Mick Jagger's quitting until the last nickel is out of the hand of the last sucker.”

“The Rolling Stones are immortal,” said John Barnes, 64, who flew to Chicago from Oxford, England, to see all three shows here. Wearing a lime green Stones lips-logo shirt and red pants, Barnes said he’s seen the band more than 250 times, starting with a 1963 show in England, and assumes the Stones will outlast him. “Unfortunately I won’t be around to see them keep playing.”

Yet let’s face some facts:

Drummer Charlie Watts turned 72 on Sunday and was treated for throat cancer in 2004.

Guitarist Ronnie Wood turned 66 on Saturday, and the UK’s Daily Mail counts eight times that he’s been in rehab for alcohol abuse, most recently in 2010.

Guitarist Richards is 69, and … do we really need to get into how he’s lived those years? Hard drug use, drinking, and he required cranial surgery after reportedly falling out of a tree in Fiji in 2006.

Jagger will turn 70 next month and is an amazing physical specimen, actually.

This may be the Stones’ “50 & Counting” tour, but how much higher could they realistically count? We’re not talking about old blues guys sitting on stools plunking out riffs. We’re talking about massive tours, lengthy sets and music that’s meant to get people to work up a sweat and to move, on and off the stage — all while the band tries to live up to a history of well-documented performances.

Chicago psychiatrist Kelly Belcher said she last saw the Stones at the Akron Rubber Bowl in the early 1970s and decided to celebrate her 60th birthday by attending Monday’s concert and enjoying musicians about a decade older than she put on a “very physical” performance.

“I’m impressed, but I’m not surprised because we of the Boomer age are not going down without a fight,” Belcher said.

If anyone expected a valedictory air to the Stones’ final “50 & Counting” Chicago performance, the Stones weren’t playing along. The show unfolded similarly to the previous two United Center shows, opening with the same quartet of songs — a punchy “Get Off of My Cloud,” a politely crunchy “It’s Only Rock ‘n’ Roll (But I Like It),” a somewhat thudding “Paint It, Black” and the extended roaring takeoff of “Gimme Shelter” — before throwing in a trio of curveballs: the fan-requested “Sway,” with guitar Mick Taylor reprising his titanic guitar leads from this “Sticky Fingers” gem; the snarling “Let It Bleed” rocker “Live with Me” and the early ballad “As Tears Go By,” which had guest Taylor Swift duetting with Jagger without quite finding a way into the song. Where Marianne Faithful brought understated melancholy to the 1965 original, Swift offered happy-to-be-here belting that sounded especially high-pitched next to Jagger’s growl.

The rest of the concert followed the script of previous shows, with Taylor returning to set the house ablaze on the blues workout “Midnight Rambler” and Richards and Wood resembling wily old bluesmen as they stomped out an acoustic “You Got the Silver,” with Richards’ craggy voice sounding no worse for wear even as he’s starting to bear a strange resemblance to Yoda. (The set included five of the nine songs on “Let It Bleed.”)

Richards’ electric guitar was turned up loud and had the tone of a cat getting its tail caught in a gate. He provided vibe while Wood did more heavy lifting with his snaky leads, Watts smacked the snot out of his snare and bassist Darryl Jones provided fluidity beneath the sometimes messy rumble.

The band never gave any indication that this night was different from all other nights. Jagger offered: “I want to thank you for coming for so many years. We’ve been coming here since 1965.” But there were no more special guests or final farewells beyond the usual bowing and waving.

Turns out, although time waits for no one, what the Stones appeared to be providing for all that money wasn’t a last hurrah so much as a message to all of those similarly getting-on-in-years folks out in those expensive seats: It doesn’t have to end.

“They’re ageless,” a gleeful Belcher said toward the end of the show, “and for a short period of time they make us feel ageless too.”